1. Field of the Invention
This invention relates to printed circuit boards. More particularly, this invention relates to protective encapsulation of printed circuit boards. Still more particularly, this invention relates to the application of dry film photosensitive solder mask to printed circuit boards and its use.
2. Description of the Prior Art
The application of liquid or dry film resist materials to smooth surfaced elements is well known. However, when it is desirable to apply a resist material to a surface having raised discontinuities, e.g., a solder mask or gold tab resist to a printed circuit board, simple lamination methods generally result in poor conformation of the resist around the raised discontinuities.
A solder mask is a protective coating which is selectively applied to portions of a printed circuit board surface to confine solder to pad areas on the board and to prevent bridging between conductors during tinning operations and during soldering of components. A solder mask also functions to prevent or minimize corrosion of the bare copper conductors and as a dielectric to insulate certain components from adjacent circuitry. Since a solder mask remains on the finished circuit board, it usually is transparent to allow for ease of circuit inspection.
A solder mask is conventionally applied as a curable ink by screen printing techniques. However, screen printing over raised circuitry on a circuit board poses several problems particularly when circuit density is high and the board is large. The curable ink must flow; yet if too much flow occurs, there is "shadowing", or flow, of the ink into holes and into areas intended to be open. Misregistration can also result in a conductor adjacent to a pad being left open with subsequent bridging of solder in the tinning operation or in soldering components.
The problems of the screen printing technique can be overcome by the use of dry, photosensitive, resist film similar to those disclosed in Celeste, U.S. Pat. No. 3,469,982, Celeste, U.S. Pat. No. 3,526,504, and French Pat. No. 7211658 as a solder mask using the process described in the aforementioned U.S. Pat. No. 3,469,982 or apparatus similar to that in Heiart, U.S. Pat. No. 3,404,057 and Cohen et al., U.S. Pat. No. 3,547,730. Another advantage of the photosensitive, dry film, solder mask lamination technique over the screen printing technique is the reduced time and ease of preparation of the masked circuit boards, since protracted curing times at elevated temperatures are avoided. The major drawback of the dry film lamination method is the inability of the film to conform around all of the raised circuitry. In particular, when a circuit board is laminated with a dry film at elevated temperatures, raised circuit components which are perpendicular to the direction of lamination are "tented" over, thereby entrapping air between the board and the mask. Such entrapped air results in poor adhesion of mask to board, blistering of the mask during tinning and soldering procedures, and loss of circuit definition when the circuit itself consists of a low melting conductor such as solder.
Similar conformation problems arise when a dry photosensitive resist film is used as a gold tab resist. A gold tab resist material is used during the manufacture of circuit boards to isolate the connector tab region of the board, so that the exposed connector contact elements of the board can be plated with gold. The tab, with gold plated contact elements thereon, then forms the electrical connector for the completed circuit board with soldered components thereon. Gold tab resist is currently applied manually as a specialized adhesive tape to all areas of a circuit board except the tab regions. The operation is meticulous and the tape material by necessity must withstand etching and plating baths.